One of those mornings, I guess. I fired up the HW-100 and scope to continue checking signals when I discovered a complete lack of the carrier oscillator frequency where I expected it. I walked back through stages with no success.
Back to basics, I guess. Stuck the scope probe on the carrier oscillator tube V16 grid where the carrier oscillation is developed, Good there. Followed it to the balanced modulator. Good there. Found it nulled out on LSB and USB, otherwise developed across transformer T1 on CW and Tune. All as it should be.
Hooked up to the cathode of the isolation amplifier V2 and found no signal. Transformer T1 output is an internal LC network with the coil tunable, coupled through C22 to the tube cathode. Zero signal on the far side of C22, so time to investigate that component and get myself hooked to the transformer output side.
|Where carrier oscillator signal disappears|
I yanked the tube out just to confirm that this is not a problem within the tube. No change. I pulled the capacitor C22 and validated its value of 24 pf. I measured the resistor and it is almost exactly 470 ohm. Nothing makes sense here.
I wonder if I am loading down the coil and circuit, changing the resonant frequency of T1 and blocking the signal? Time to clip the probe near but not on the line, resulting in much less capacitive coupling. Unfortunately, not enough signal to detect this way.
I did discover that the carrier null potentiometer was dirty and noisy, easily switching its resistance. Putting some Deoxit inside and working it a bit lead to a much more stable control, that allowed me to null the carrier and keep it that way.
The output of the isolation amplifier V2 was then monitored, which should give me the same signal at higher amplitude, but it was in fact weaker. This circuit is a grounded grid amplifier (see pin 2 above, the suppressor grid), with the input signal on the cathode and the varying potentials on the control and screen grids determining the degree of amplification.
The control grid is fed from the ALC (automatic level control) circuit and the screen grid is controlled by the CW/Mic gain circuit. The B+ to the screen grid is switched on only during transmission (including Tune mode), and read about 150V. The control grid potential was -15 to 0 depending on the MIC/CW gain control while running in CW or Tune mode.
Something is still wrong, I get almost no carrier oscillator amplitude into and out of the isolation amplifier V2. Still haven't figured out the issue but will keep searching. Perhaps I should check the four diodes in the modulator, as these have a history of going bad in similar transceivers. Just in case I ordered some spare diodes from Digikey.
Meanwhile, I decided to go through the 'voltage charts' provided by Heath, for the unit in both transmit and receive modes. I couldn't get into the one section because all the driver coils and crystals are covered by a RF shield plate, and it wasn't worth the problem to check there yet.
Everything matched up except for one voltage, which turned out to be the crystal calibrator oscillator plate. That is not switched on unless the PTT-VOX-CAL switch is placed in the CAL mode, which it was not. If the chart had identified that dependency I wouldn't have spent the five minutes researching the 'missing' voltage in the schematics.
I went over all the modifications I made to the audio, modulator, if and bandpass boards, carefully checking the work for errors, bridges or loss of continuity. Good to know that they were all correctly installed. A few were right at the carrier oscillator, but those changes lowered the plate resistors slightly to increase signal strength. Nothing should be causing the symptoms I face.
The ALC system lowers amplification on several different tubes and is probably the reason why I have almost no signal strength. I will spend the remainder of the evening studying the ALC circuits and determining what I should be seeing at various tube grids, for when I test again tomorrow.